1. Field of the Invention
The present invention relates to an optical device that provides a metal cap welded to stem to seal a space air-tightly where a semiconductor optical device, such as semiconductor laser diode (LD) or semiconductor photodiode (PD), is mounted therein.
2. Related Prior Art
An optical device that provides a co-axial package, generally comprises of a metal stem and a metal cap. The stem mounts the LD or the PD and occasionally some electronic components such as pre-amplifier IC for amplifying a signal generated by the PD and driver IC for driving the LD. The cap, fixed to the stem air-tightly, provides a space in which the LD or the PD and the electronic components are enclosed. In the co-axial package of the optical device, the stem has a disk shape, while, the cap has a cylindrical shape. The cap comprises a shell portion and a flange portion. The shell portion provides a transparent window through which the light emitted from the LD or heading the PD installed within the space passes; while the flange portion located in a side opposite to the transparent window is fixed to the stem by the welding.
Assembling the cap with the stem, the cap in the flange portion thereof is fixed to the stem by, for instance, the resistance welding. As illustrated in FIG. 4, the resistance welding puts the flange 102a of the cap 102 aligned with the stem 103 between two electrodes, E1 and E2, such that a peripheral portion of the flange 102a and the stem 103 are pressed by the electrodes E1 and E2 and provided with a huge pulse current to melt the peripheral portion of the flange 102a and the stem 103. Thus, the cap 102 is welded to the stem 103. However, the resistance welding described hereto often causes or brings a failure such as short-circuit between components or mechanical damages induced in the components installed within the package by fragments melted at the welding and flown within a inner space. Even the welding does not bring the fragment flown in the space S, the surplus metal melted by the welding seeps out within the inner space S and occasionally causes the flown fragment when the cap 102 and the stem 103 receives vibrations.
The flange portion 111a of the shell 111 sometimes provides a ringed projection 111b with a height about 0.1 mm to increase the current density at the welding. This ringed projection 111b may facilitate the melting of the flange portion 111a and makes the welding in homogeneous around the stem 112. However, the ringed projection 111b brings no effect for the detachment of excess portions of the projection into the space S. The top end portion of the ringed projection 111b easily melts by the pulsed current, rather, the melting is facilitated by the projection, the melted projection easily expands within the space S by the pressure to abut the flange portion 111a against the stem 112.
The melted projection and extended within the space S causes the failures mentioned above. An adequate condition of the resistance welding, in particular, reduced pressure to abut the flange portion 111a against the stem 112, may decrease the possibility for excess fragments by the welding to ooze within the space S. However, the reduced pressure at the welding inevitably brings a less reliability in the air-tightness.
A Japanese Patent Application published as JP-2007-134644A has disclosed an optical device with a cap welded to the stem, which is schematically illustrated in FIG. 7. The flange portion of the cap in the prior art above provides a ringed projection 122a accompanied with a step 122b extending along and immediate inside of the ringed projection 122a. The step 122b, which is provided to secure the height of the welded projection 122a and has a function to protect the projection 122a against the excess crushing, may also show another function to prevent the melted fragments by the welding from flying into the space S.